2.1 Field of the Invention
The exemplary, illustrative, technology herein relates to Solid Oxide Fuel Cell (SOFC) systems, methods, and methods of manufacturing. In particular the exemplary, illustrative technology relates to manufacturing and using solid oxide fuel cells.
2.2 The Related Art
Fuel cells are used to generate power by an electrochemical process. The components that generate power are commonly referred to as “cells.” As the voltage for an individual cell may be relatively low, it is often necessary to connect a plurality of cells, either in parallel or in series, to provide power to a device having a desired operating voltage. Such an array of cells is generally referred to as a “stack” or “bundle.”
Solid oxide fuel cells (SOFCs) are a particularly useful type of fuel cell because they can operate on an expanded list of fuels, which includes pure hydrogen but also includes hydrocarbon fuels such as propane, gasoline, diesel, kerosene (JP-8 military fuel), ethanol, and other bio-fuels. SOFCs use a ceramic electrolyte and generate current when oxygen ions cross the electrolyte. However, one drawback of SOFCs is the need to operate the fuel cell at high temperature (e.g. above 500° C.) to process the fuels. The high temperature operation leads to a variety of problems, including thermal creep and increased susceptibility to corrosion, which creates the need to select heat resistant materials (e.g. ceramics, Inconel, and other high temperature metal alloys) for such components as the fuel cell stacks, structural elements supporting the fuel cell stacks, and various gas seals.
Referring now to FIGS. 1A and 1B, an example conventional SOFC stack (100) comprises two cup shaped opposing end plates (110) with multiple electrolyte rods (150) supported between the opposing end plates. Each rod comprises a ceramic electrolyte, formed from nickel oxide or the like, and each rod acts as a single fuel cell in a fuel cell stack (100) that comprises a plurality of rods. Each rod (150) comprises a hollow conduit (152) surrounded by an outer wall (154) with the conduit having opposing open ends (156). Each conduit conveys a gas or vapor fuel from an input end to an output end. The exterior surface (158) of each rod is coated with a cathode material and the interior surface (159) of each rod is coated with an anode material. One example conventional SOFC device is disclosed in commonly assigned U.S. application Ser. No. 12/367,168 now U.S. Pat. No. 8,304,122 to Poshusta et al., filed on Feb. 6, 2009, entitled Solid oxide fuel cell system with hot zones having improved reactant distribution which is incorporated herein by reference in its entirety.
Fuel is simultaneously flowed through each of the hollow conduits (152) as an oxidant, (e.g. air or another oxidant) is flowed over the external surfaces (158) (e.g. by a fan, blower, natural convection, or the like).
Alternatively, conventional solid oxide fuel cells are made with a cathode coating on the interior of the cell (159) and an anode coating on the exterior of the cell (158). These cells operate with the oxidant flowing through the interior of the rods while fuel passes over the external surface.
In operation the rods electrochemically react with the fuel flowing over the anode (e.g. through the conduit) (152), and with the oxidant passed over the cathode (e.g., the outer surface of the rods) (158). The electrochemical reaction generates a current flow along longitudinal surfaces of each rod. Each of the rods is connected to an anode terminal at a first end and a cathode terminal at an opposing end (not shown) as required for delivering the current flow generated in each rod out of the fuel cell.
In some conventional SOFC systems, an electric heater or burner burning the same fuel being delivered to the rods is disposed to initially heat the rods from ambient temperature to an operating temperature range of 650-1000° C. in order to initiate the desired electrochemical reaction. Thereafter the heater or burner may not be required as the electrochemical reaction is exothermic and produces sufficient heat to activate and maintain subsequent reaction.
In a conventional SOFC system, the rods are captured between the opposing end plates (110) which support the rods in an operating position, such that the rods are oriented parallel to each other in a bundle. The end plates also serve as input and output manifolds functioning as end plates of a cylindrical cathode chamber can, which encloses a cylindrical volume between the end plates. Additionally a feed can (not shown) is disposed on an inlet side of rod stack wherein the input end plate acts as a manifold between the feed can and the cathode chamber to prevent fuel from entering the cathode chamber. In particular the input end plate directs fuel through the hollow portion of each rod where it reacts with anode materials disposed on internal surfaces (159) of the rods. Meanwhile oxidant flows through the cathode chamber rod where it reacts with cathode materials disposed on external surfaces (158) of the rods. Accordingly there is a need to seal the flow can at each rod/endplate interface to prevent oxidant from escaping the cathode can and to prevent fuel from entering the cathode can. The end plates (110) may also serve as conductors for conducting current from each fuel cell rod to appropriate electrical terminals (e.g. to power connected electrical loads). In particular opposing end plates (110) may comprise anode and cathode conductors respectively. Moreover, it is desirable to electrically connect the anodes and cathodes of each rod in series so there is a further need to electrically isolate rod external surfaces from rod internal surfaces at the interface between each rod end and the end plates. Accordingly there is a need to electrically isolate the rod/endplate interface to prevent electrical shorts between the rod anode and cathode surfaces.
One problem with conventional SOFC designs is that it is difficult to attach the ceramic rods (150) to the metal end plates (110) without gas leaks developing at the rod end plate interface after the system reaches a steady state operating temperature (e.g. between 650-1000° C.). This is because the metal end plates and the ceramic rods have different thermal expansion characteristics and thus expand at different rates with increasing temperature. This non-uniform thermal expansion leads to gaps developing at the interface between the rods and the end plates, causing gas leaks, especially after the system reaches its operating temperature.
Another problem with the rod-to-plate interface is that the materials tend to permanently deform over time as a result of thermal creep which can occur after prolonged high temperature operation even when the stresses involved are below the yield strength of the material. Thermal creep is especially severe in materials subjected to high temperature environments for long periods. Moreover the ceramic rods and metal end plates have different creep characteristics. In the example of the conventional rod end plate interface, the metal end plates (110) are more likely to become permanently deformed at operating temperatures than the ceramic rods because creep in the metal end plates occurs at a lower temperature than creep in the ceramic rods.
Another problem with conventional SOFC systems is that the high temperature operation leads to excessive oxidation of the metal end plates as a result of constant exposure to the oxidant. Over prolonged use, a buildup of oxidation degrades electrical conductivity across the end plates and may lead to further gas leaks.
The rod to end plate interface problem has been addressed in conventional SOFC fuel cells by potting the rods in place using a high temperature ceramic potting material such as Aremco's Ceramabond 552 and 668, or the like. More specifically each end plate (110) is formed cup-shaped with a circular base wall (111) and an annular side wall (112) extending toward the opposing end plate. Each base wall (111) includes a plurality of through holes (115) with each through hole sized to receive a rod end there through. Thus the rods (150) are installed through the base walls (111) with a slight protrusion at each end and the annual side wall height is about 0.1-0.2 inches. Conventionally the rod ends have a clearance fit with respect to holes passing through the end plates. The end plates with installed rod ends are filled with the liquid ceramic potting material (160) which is then cured. In another embodiment, each end plate may include a circular ceramic plate that fits over the outer diameter of each rod and mates with the annular side wall such that liquid ceramic potting material is poured between the end plate and circular ceramic plate and cured to create a seal. The cured potting material (160) seals any gaps between the rods and base wall and mechanically supports each rod in a desired operating position. Thus the potting material (160) overcomes the problems associated with expansion and creep in the metal end plates by sealing each end of the ceramic rods for a length of about 0.1 to 0.2 inches.
One problem with potting the rod in places is the cost of labor to pot the end plates and the cost of the potting material.
A second problem with potting the rods in place is that the cured potting material is very stiff and the rods, which are made from brittle, inflexible ceramic materials such as nickel oxide, can break or crack when the rod assembly is bent or twisted during insertion of the rod assembly into its housing. Any cracks in the rods will result in a fuel leak. Moreover due to the permanent attachment of the end plates to the rods by potting, even if only one rod cracks the entire rod assembly becomes unusable and must be replaced.
A third problem with potting the ends of each rod is that the potted ends do not participate in the electrochemical reaction which reduces the effective length of each rod. This leads to an overall lowering of current generating capacity of each rod of the rod assembly.
Therefore, there is a need for an SOFC end plate seal that ensures a strong and robust mechanical connection between the rod ends and the end plates and a mechanical connection that is less susceptible to rod damage due to bending or twisting forces applied to the rods during assembly and handling, Meanwhile there is still a need to reliably gas seal the rod end plate interface at operational temperatures (e.g. between 650-1000° C., for example at or around 750° C.), to prevent electrical shorts between anode and cathode surfaces and to provide good electrical terminal contact at each rod end to fully utilize current generated in each rod. There is a further need avoid gas leaks from occurring after prolonged use such as may be caused by dissimilar thermal creep associated with the metal end plates and the ceramic rod ends and or caused by excessive oxidation. Additionally, a desirable end plate seal reduces cost and may allow disassembly and reassembly of individual rods.